1. Field of the Invention
This invention relates to hot working die steel for use in relatively high temperature and members for high-temperature use comprising the hot working die steel, such as a casting die, a structural member for a casting machine, an injection die, a structural member for an injection molding machine, a hot forging die, an extrusion die, and the like.
2. Related Art
Where a light metal or an alloy mainly comprising a light metal (hereinafter inclusively referred to as light metal), such as aluminum, an aluminum alloy, magnesium or a magnesium alloy, or a low-melting metal or an alloy mainly comprising a low-melting metal (hereinafter inclusively referred to as low-melting metal), such as lead, a lead alloy, tin or a tin alloy, is manufactured by casting, JIS-SKD J61 steel of 5% Cr type has been used as a casting die or a structural member of a casting machine that is to be exposed to high temperature. Injection molding has been recently introduced in the manufacture of the light metals or low-melting metals, and JIS-SKD 61 steel is also used as an injection die or a structural member of an injection molding machine. Additionally JIS-SKD 61 steel has generally been applied to hot dies for casting steel applied to hot dies for casting steel materials.
The life of JIS-SKD 61 steel in these uses is ended by various causes, which are roughly divided into the following three factors. The first factor is softening due to long-term use in high temperature. JIS-SKD 61 steel has a structure of tempered martensite in which carbides are finely dispersed to serve for strengthening. However, while it is exposed to high temperature for a long time, dislocations restore, and the carbide grains agglomerate to become coarse. As a result, the material fails to retain its initial characteristics and is gradually softened. The second factor is cracks called a heat check. A heat check is cracks occurring on the material surface in a tortoiseshell pattern, which can be seen as ascribed to cyclic abrupt heating and cooling. The third factor is a melt loss phenomenon. Since a molten metal or alloy is highly reactive, the material surface in contact with a molten metal or alloy gradually undergoes denaturation and consumption.
Thus, JIS-SKD 61 steel that has been used conventionally tends to have poor durability on account of its insufficient resistance against high temperature softening, a heat check and a melt loss.